Inflatable packers that are in common use in the oil exploration and workover industry have an elongated internal elastomer sleeve that is surrounded by protective armor, for example circumferentially spaced, overlapped metal slats, reverse-layed cables, or composite constructions such as woven cables or wires. Such armor is designed to protect the elastomer sleeve from abrasions and cuts as it is expanded outward by fluid under pressure. An external elastomer sleeve may surround all or part of the armor to provide a seal against a surrounding well bore wall. A single inflatable packer can be used to provide a bridge plug in the well bore, or a straddle arrangement of upper and lower inflatable packers can be used to perform well service operations off bottom.
The upper and lower ends of the armor usually are attached by welding or the like to collars on the packer mandrel to form a unitary assembly. The opposite end portions of the armor assembly extend underneath stress rings which are mounted adjacent the collars. When the packer is expanded to its full diameter, large hoop stresses are generated in the stress rings by outward pressure of the armor end portions which may curve outward at a fairly sharp radius. Thus the strength of such stress rings is a design consideration of high importance in the successful operation of an inflatable packer.
In the past, such stress rings have been machined from conventional metal bar stock. Although increased strength of the machined stock can be achieved by cold-working, optimum strength cannot be achieved using relatively large bar (for example greater than 1.5 inches diameter) due to practical cold working limitations. Moreover, cold-worked bar properties are optimum along the axis of the bar, while the stress rings of an inflatable packer experience high hoop stresses on account of their radial loading by the end portions of the armor. Thus a stress ring made in a conventional manner has a tendency to crack and split in radial directions and cause downhole packer malfunctions which are highly undesirable.
A general object of the present invention is to provide a new and improved inflatable packer having stress rings which are manufactured in a way such that the stress rings have optimum strength in view of the principle stresses that are generated therein as the packer element is expanded.